Rename Framebuffer to Canvas (issue #263). Move error reporting code.

Error reporting now resides in Graphics::newCanvas() instead of the
previous location wrap_newFramebuffer(). Changed error text for
"Error in implementation" to point to a possible fix.

src/modules/graphics/opengl/Graphics.h

@@ -41,7 +41,7 @@
 #include "Quad.h"
 #include "SpriteBatch.h"
 #include "ParticleSystem.h"
-#include "Framebuffer.h"
+#include "Canvas.h"
 #include "PixelEffect.h"
 
 namespace love
@@ -287,7 +287,7 @@ namespace opengl
 
 		ParticleSystem * newParticleSystem(Image * image, int size);
 
-		Framebuffer * newFramebuffer(int width, int height);
+		Canvas * newCanvas(int width, int height);
 
 		PixelEffect * newPixelEffect(const std::string& code);